Fix autogen.sh’s spurious ‘git diff’ output

Fix autogen.sh’s spurious ‘git diff’ output

Problem reported by Gerd Möllmann (Bug#68464).
* .gitignore: Ignore files in exec that are now copied from build-aux.
* admin/merge-gnulib (avoided_flags): Instead of clearing
autom4te.cache here ...
* autogen.sh (do_git): ... clear it here.
Use config.guess, config.sub and install-sh from
the Emacs repository, as they are more likely to be up to date.
This avoids unnecessary differences among different builders,
and avoids unnecessary ‘git diff’ output after autogen.sh.
Also, copy these files from build-aux to exec since there
should be no difference between the two copies.
* exec/config.guess, exec/config.sub, exec/install-sh:
Remove from repository, as autogen.sh now copies them from build-aux.